porcelain.status() output different type of file names.

porcelain.status() output untracked file as string when others output as bytes.
It looks like:
```
>>> porcelain.status('./')
GitStatus(staged={'add': [b'a'], 'delete': [b'README.md'], 'modify': [b'demo.py']}, unstaged=[b'requirements.txt'], untracked=['b'])
```
